1. Field of the Invention
The present invention relates to an inverter circuit for a discharge tube for lighting and driving a discharge tube such as a cold-cathode fluorescence tube, a hot-cathode fluorescence tube, a mercury lamp, a sodium lamp, a metal halide lamp, or a negative glow lamp.
2. Description of the Prior Art
Lighting of the discharge tube requires both of a high-voltage power supply such as commercial power supply system and a lightening circuit consisting of a ballast. In recent years, an inverter circuit is used for obtaining a high voltage power supply from a low voltage DC power supply, for the purpose of miniaturization of the lightening circuit or for the purpose of dissemination of a portable type equipment.
Conventionally, as shown in FIG. 9, this kind of inverter circuit is generally used. The inverter circuit comprises a pair of transistors Q.sub.1 and Q.sub.2, a step-up transformer T having a primary winding L.sub.1, a secondary winding L.sub.2, and an auxiliary winding L.sub.3. The collectors of transistors Q.sub.1 and Q.sub.2 are connected to the both sides of the primary winding L.sub.1 of the step-up transformer T, the emitters thereof are interconnected each other, and connected to ground. Further, the intermediate point of the primary winding L.sub.1 is connected to the bases of the transistors Q.sub.1 and Q.sub.2 through the resistances R.sub.1 and R.sub.2 and to each end of the auxiliary winding L.sub.3 of the step-up transformer T. A collector resonance type high-frequency oscillating circuit OS of the inverter circuit is composed of the primary winding L.sub.1 of the step-up transformer T, the capacitor C1 which is connected parallel thereto, the transistors Q.sub.1 and Q.sub.2, and the auxiliary winding L.sub.3 and the like.
One terminal of the secondary winding L.sub.2 of the step-up transformer T is connected to one end of the discharge tube DT through the ballast capacitor C.sub.2 and electrical wiring L, and the other terminal thereof is connected to the another end of the discharge tube DT and to ground. Further, C.sub.3 is parasitic capacitance of the secondary winding L.sub.2, and C.sub.4 is parasitic capacitance at periphery of the discharge tube DT.
In the case of the above-described inverter circuit, the step-up transformer takes up the largest space in regard to the circuit. Since it is difficult to miniaturize the step-up transformer, it is incapable of being diminished the shape of the whole inverter circuit. When it allows the driving frequency to increase, the miniaturization of the step-up transformer can be achieved. However, the following method also makes it possible for the whole inverter circuit to miniaturize.
In the above-described conventional circuit, since the circuit is only connected from the high-impedance load to the low-impedance load through the capacitance ballast, an impedance of load as seen from power supply side of high-impedance is hardly matched with an impedance of power supply side as seen from load side. For this reason, when the driving frequency is increased, a reflection is generated in the side of the load, so that a part of supplying capability returns to the side of power supply.
As shown in FIG. 10, caused by a mismatching of the impedance, phase between voltage and electric current is shifted so that the power supply can not be used efficiently. The electric power which returns to the prior stage is increased, following this, dielectric current is increased. Accordingly, copper loss or dielectric loss is increased depending upon increasing of the reactive current, there occurs the problems that conversion efficiency of the electric power is lowered. The value which is obtained by multiplying a voltage root mean square value by a current root mean square value does not come into the electric power which is provided at the discharge tube.
Furthermore, when the driving frequency is increased, the value of the ballast capacitance C.sub.2 is diminished from the view point of the design, with the result that the ratio of parasitic capacitance C.sub.3 corresponding to the ballast capacitance C.sub.2 becomes large so that it causes the supply voltage to the discharge tube DT to lower, thereby lighting luminance of the discharge tube DT is lowered. In particular, in order to use the discharge tube as a light source for liquid crystal back light, when the reflection member made of the electrically conductive sheet which is formed in such a way that the polyethylene telephthalate film is subjected to sputtering of silver, the parasitic capacitance at periphery of the discharge tube is further increased. The parasitic capacitance at periphery of the discharge tube causes the applied voltage to the discharge tube to lower so that the lighting luminance of the discharge tube DT is greatly lowered.
This phenomenon is similarly generated when the piezo-electric transformer is employed as a step-up transformer. Between a characteristic capacitance which is corresponding to the ballast capacitance C.sub.2 involved as the equivalent circuit into the piezo-electric transformer and the parasitic capacitance C.sub.3, the same voltage dividing effect as the conventional winding transformer is generated, this causes the burning luminance of the discharge tube DT to lower. Lowering of lighting luminance by the electrical conductive reflection sheet can not be avoided in the piezo-electrical transformer, therefore, in order to lessen the voltage dividing effect, there is a problem that it allows the shape of the piezo-electrical transformer to magnify so that it allows the characteristic capacitance C.sub.2 to increase.